Natriuretic peptides modulate salt and water homeostasis in the body and in this way act as regulators of blood pressure. The peptides belonging to this family have varying amino acid sequences and are secreted through different mechanisms by various tissues in the body. These peptides include atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP). These peptides bind to three types of receptors that signal intracellularly to modulate physiological functions. ANP and BNP bind preferentially to natriuretic peptide receptor A (NPR-A) (also known as guanylyl cyclase A (GC-A)), and CNP binds preferentially to natriuretic peptide receptor (NPR-B) (also known as guanylyl cyclase B (GC-B)). All three peptides have similar affinity for natriuretic peptide receptor C(NPR-C), which has both signaling and peptide clearance functions. Clearance of natriuretic peptides also occurs through the action of membrane-bound neutral endopeptidase (NEP).
Peptide binding to NPR-A or NPR-B activates the intracellular guanylyl cyclase domain of these receptors, which produces the second messenger cGMP. cGMP activates or inhibits multiple signaling pathways inside the cell.
Bone formation and longitudinal bone growth in long bones, ribs, and vertebrae occurs via endochondral ossification in the cartilaginous growth plate, which is located at both ends of the bone. One important regulator of bone growth is CNP, which circulates in blood at a very low level, suggesting that it has very little systemic activity on bone. Studies using primary cultures of osteoblast-like cells and chondrocytes have revealed that CNP acts rather as a paracrine/autocrine factor to regulate proliferation and differentiation of osteoblasts and chondrocytes. CNP, through activation of NPR-B guanylyl cyclase, stimulates the production of intracellular messenger cGMP in chondrocytes and is important for bone growth and development.
In humans, CNP is initially produced from the natriuretic peptide precursor C(NPPC) gene as a single chain 126-amino acid pre-pro polypeptide. Removal of the signal peptide yields pro-CNP, and further cleavage by the endoprotease furin generates an active 53-amino acid peptide (CNP53), which is secreted and cleaved again by an unknown enzyme to produce the mature 22-amino acid peptide (CNP22). Both CNP53 and CNP22 bind similarly to NPR-B, and they both induce cGMP production in a dose-dependent and similar fashion.
Genetic deletion of CNP, its cognate receptor, or downstream intracellular effector (PKG) results in severe skeletal dysplasias caused by reduced chondrocyte proliferation and differentiation. In mice lacking CNP, dwarfism and early death occur. At birth, these mice have approximately 10% reduction in bone length, but the growth retardation becomes more severe postnatally, and 70% of the mice die in the first 100 days after birth. Cartilage-specific overexpression of CNP partially rescues the achondroplasia dwarfism of the CNP-deficient mice, suggesting that CNP stimulates bone growth through direct effects on chondrocytes. Functional inactivation of the natriuretic peptide (NPR)-B receptor that binds CNP or gene encoding for cGMP protein kinase II through which cGMP effects are mediated also produces dwarfism.
Skeletal dysplasias are a group of genetic disorders characterized by impaired skeletal growth. The many different forms of skeletal dysplasia, e.g., short limb dwarfism, are associated with significant morbidity and mortality. Achondroplasia is the most common form of short limb dwarfism in human beings, affecting more than 250,000 individuals worldwide. Achondroplasia is caused by mutations in the gene encoding fibroblast growth factor receptor 3 (FGFR3), which cause gain of FGFR3 function. These mutations affect many tissues, but most strikingly the cartilaginous growth plate in the growing skeleton, leading to a variety of manifestations and complications. The severity of the clinical phenotype is related to the capacity of the mutation to overactivate FGFR3 signaling pathways in chondrocytes.
The intracellular production of cGMP resulting from NPR-B activation is known to inhibit the MAP-kinase pathway overactivated by the FGFR3 mutation. Thus, use of CNP or a CNP analog that could activate the NPR-B signaling pathway for the treatment of skeletal dysplasia has been considered. However, a major drawback of the therapeutic use of CNP is its extremely short half-life. Furthermore, experiments in the literature, e.g., Farnum et al. (Anat. Rec. A Discov. Mol. Cell Evol. Biol. 288(1):91-103, 2006), have shown that the ability of a molecule to enter the growth plate decreases significantly with molecular weight, with poor or no detectable entry by molecules of 40 kDa or larger into the growth plate. Accordingly, it has been believed that CNP-based therapeutics for skeletal dysplasias such as achondroplasia need to have a relatively small molecular weight in order to be capable of entering the growth plate at a sufficient rate to have a therapeutic effect.
There is thus a need in the art to develop therapeutic molecules having an appreciable half-life and other favorable pharmacokinetic and therapeutic properties for the treatment of a variety of disorders, such as achondroplasia.